In the field of mobile communication, research and development of next generation mobile communication systems are being conducted at a rapid pace. In such mobile communication systems, signals are transmitted as packets in unit transmission periods called transmission time intervals (TTIs). The transmission time interval (TTI) may also be called a time period or a subframe. The TTI is used as a base unit of various types of signal processing such as radio resource allocation, channel coding, and retransmission.
Meanwhile, a period of time from when a data channel including user data is transmitted from a sending end to a receiving end where the data channel is demodulated and decoded until when the sending end receives delivery confirmation information (acknowledge or negative acknowledge) from the receiving end is called a control delay or a round-trip delay. For a real-time application (such as a multiplayer game) or to increase transmission control protocol (TCP) throughput, it is preferable to use a short TTI and thereby to reduce the control delay.
On the other hand, as described below, it is preferable to use a long TTI to increase the area of a cell in a mobile communication system where, in uplink communications, a base station receives relatively high power from a user device near the base station but receives relatively low power from a user device at a cell edge.
FIG. 1 shows packets with short and long TTIs. Both the short TTI packet (left) and the long TTI packet (right) include a control channel labeled as “Control” in FIG. 1. The short TTI packet also includes a data channel labeled as “Data”. The long TTI packet also includes data channels labeled as “Data 1” and “Data 2”. The two portions labeled as “Control” in the long TTI packet include the same control channel. This means that the long TTI packet can use a higher level of power for a control channel than the short TTI packet. Accordingly, to increase the area of a cell, it is preferable to use a long TTI and thereby to improve the quality of uplink signals (particularly, control channels). The data channels “Data 1” and “Data 2” in the long TTI packet contain different information but are encoded by the same coding scheme since they belong to the same TTI. Unlike data channels, it is difficult to improve the quality of control channels by techniques such as retransmission control and adaptive modulation and coding (AMC). Therefore, transmission power and TTI are important parameters for improving the quality of control channels.
Thus, it is preferable to determine the length of TTI depending on communication environments and purposes. For example, TR-25.896 V6.0.0 2004-03 Feasibility Study for Enhanced Uplink for UTRA FDD (Section 8.2) proposes a method where an appropriate TTI is selected from a set of predefined TTIs with different lengths in a mobile communication system.